Coherent laser radar device

ABSTRACT

An object of the invention is to obtain a coherent laser radar device that provides one system of a photo detector and eliminates a blind zone, including a pulsed laser that oscillates at a frequency which is identical with or close to an output light of a local light source as a single frequency; a transmission/reception optical system that irradiates a pulsed laser beam from the pulsed laser toward a target as a transmission light and receives a scattered light from the target as a reception light; a light coupling means that couples the output light from the local light source and the reception light; a photo detecting portion that conducts light coherent detection on a coupled light; and a signal processing device that calculates a speed and a distance of a target in accordance with an output of the detection, in which the photo detecting portion comprises: a photo detecting element that conducts the light coherent detection; a microwave switch that changes over a propagation path of an output from the photo detecting element; a microwave amplifier; and a switch control means that changes over the microwave switch so as to transmit a signal before a reference time as a monitor signal and transmit a signal after the reference time as a reception signal with a time at which the pulse light from the pulsed laser has completely passed through the transmission/reception optical system as the reference time.

TECHNICAL FIELD

[0001] The present invention relates to a laser radar device, and moreparticularly to a coherent laser radar device using a pulsed laser thatoscillates at a single wavelength as a light source for the purpose ofmeasuring physical information such as a distance, a velocity, a densitydistribution or a velocity distribution of a target.

BACKGROUND ART

[0002] A coherent laser radar device using a laser beam can measure awind velocity or a wind velocity distribution even in fine weatherbecause a sufficient scattering intensity is obtained even throughaerosol existing in the atmospheres. For that reason, the coherent laserradar device is preferably located at an airport or mounted on anaircraft and expected as a device for detecting a hindrance including anair turbulence.

[0003] The coherent laser radar devices are of two types one of whichemploys a pulsed laser that oscillates at a single frequency as a lightsource and the other of which employs a CW (continuous wave) laser.

[0004]FIG. 9 is a structural diagram showing a laser radar device inwhich a coherent laser radar device using an injection seeding pulsedlaser device as a light source as disclosed in U.S. Pat. No. 5,237,331 Bby Sammy W. Henderson et al. is combined with a wavelength synchronizingcircuit for stabilizing the wavelength of the laser radar light sourceas disclosed in JP 10-54760 A by Shoji and Hirano.

[0005] The laser radar device shown in FIG. 9 includes a CW laser lightsource 1 that oscillates at a single frequency, a first optical divider2 that branches a laser beam 19 from the CW laser light source 1, afrequency shifter 3, an injection seeding pulsed laser 4, a beamsplitter 5, a ¼ wavelength plate 6, a telescope 7, a scanning opticalsystem 8, a first optical coupler 9, a photo detecting portion 10, asecond optical divider 11, a third optical divider 12, a second opticalcoupler 13, a signal processing device 16, an adjusting mechanism 17 fora cavity length of the injection seeding pulsed laser 4, and a controlcircuit 18 for the adjusting mechanism. Reference numeral 20 denotes aseed light from the frequency shifter 3, 21 is a pulsed laser beam fromthe injection seeding pulsed laser 4, 22 is an optical axis of atransmit/reception light, 23 is a transmission light, 24 is a receptionlight, 25 is a local light and 26 is a coupled light of the receptionlight 24 and the local light 25 due to the first optical coupler 9.

[0006] Subsequently, the operation of the laser radar device shown inFIG. 9 will be described. The laser beam 19 from the laser light source1 that oscillates at a single frequency f₀ is branched by the firstoptical divider 2 into two beams one of which forms the local light 25,and the other of which becomes a laser beam that increases in frequencyby a frequency f_(IF) by the frequency shifter 3 and is then supplied tothe injection seeding pulsed laser 4 as the seed light 20.

[0007] The injection seeding pulsed laser 4 conducts the pulseoscillation at the single frequency (single wavelength) in an axis modehaving a frequency closest to the seed light 20. The laser pulse 21 fromthe injection seeding pulsed laser 4 which is linearly polarized isreflected by the beam splitter 5 through the second optical divider 11.Thereafter, the reflected light is transformed into a circularlypolarized light by the ¼ wavelength plate 6 and then irradiated toward atarget through the telescope 7 and the scanning optical system 8 as thetransmission light 23.

[0008] A scattered light from the target is received through a backwardpath of the transmission light. The reception light 24 becomes alinearly polarized light shifted from a polarization plane of the laserpulse 21 by 90 degrees due to the ¼ wavelength plate 6 and is thentransmitted through the beam splitter 5 so as to be guided to the firstoptical coupler 9. In the first optical coupler 9, the reception light24 and the local light 25 are coupled together, and the coupled light 26is supplied to the photo detecting portion 10.

[0009] In this example, the photo detecting portion 10 is structured asshown in FIG. 10.

[0010] As shown in FIG. 10, the photo detecting portion 10 includes afirst photo detector 27 and a second photo detector 28. Each of thefirst and second photo detectors 27 and 28 is made up of a photodiodethat functions as a square-low detector which conducts light coherentdetection and a microwave amplifier that electrically amplifies a signalfrom the photodiode. The microwave amplifier is shown by the combinationof a pre-amplifier and a post-amplifier in the figure. A detectionoutput from the first photo detector 27 is outputted to the signalprocessing device 16 as a reception signal, and a detection output fromthe second photo detector 28 is outputted to the signal processingdevice 16 as a monitor signal.

[0011] Returning to FIG. 9, the coupled light 26 from the first opticalcoupler 9 is coherent-detected by the first photo detector 27 of thephoto detecting portion 10. A signal from the first photo detector 27 isinputted to the signal processing device 16 as the reception signal. Thesignal processing device 16 calculates a distance to the target inaccordance with an arrival period of time of the reception signal (aperiod of time since the transmission of the transmission light to thetarget till the reception of the reception light from the target,analyzes the frequency of the reception signal to obtain a Dopplersignal and extracts the velocity of the target from the Doppler signal.

[0012] As described above, the injection seeding pulsed laser 4 isrequired to monitor a difference in frequency between the pulsed laserbeam 21 and the local light 25 in order to obtain an accurate Dopplersignal since the injection seeding pulsed laser 4 conducts the pulseoscillation at the single frequency in an axis mode having a frequencyclosest to the seed light 20. For that reason, after a part of the laserpulse 21 and a part of the local light 25 are extracted as monitorlights from the second and third optical dividers 11 and 12,respectively, and then coupled together by the second optical coupler13, the coherent detection is conducted by the second photo detector 28within the photo detecting portion 10 a. A signal from the seconddetector 28 becomes the monitor signal.

[0013] In the signal processing device 16, a frequency difference (thefrequency of the monitor signal) f_(M) between the laser pulse 21 andthe local light 25 and the oscillation timing of the laser pulse areobtained from the monitor signal. Assuming that the frequency of thelocal light 25 is f₀, the respective frequencies f_(s), f_(T), f_(R),f_(M) and f_(sig) of the seed light, the laser pulse, the receptionlight, the monitor signal and the reception signal are represented bythe following expressions.

f _(s) =f ₀ +f _(IF)

f _(T) =f _(s) +Δf

f _(R) =f _(T) +f _(d)

f _(M) =f _(IF) +Δf

f _(sig) =f _(M) +f _(d)

[0014] where Δf is a frequency difference between the laser pulse 21 andthe seed light 20, and f_(d) is a Doppler frequency of the target. Adifference between the frequency f_(sig) of the reception signal and thefrequency f_(M) of the monitor signal is taken, thereby being capable ofobtaining the Doppler frequency f_(d) of the target.

[0015] In order that the injection seeding pulsed laser 4 stably obtainsthe injection seeding operation, the injection seeding pulsed laser 4adjusts the cavity length of the pulsed laser by using a piezoelectricelement as the adjusting mechanism 17 for the cavity length. Thepiezoelectric element that functions as the adjusting mechanism 17 ofthe cavity length is controlled by the control circuit 18. In the signalprocessing device 16, an error signal based on a value of the frequencydifference f_(M) between the laser pulse 21 and the local light 25 istransmitted to the control circuit 18 from the monitor signal. In thecontrol circuit 18, the cavity length of the pulsed laser 4 is adjustedby the piezoelectric element so that the value of Δf is set to be a setvalue or less, or 0.

[0016] In this way, the laser pulse that stably oscillates in a singlemode (single wavelength) is obtained.

[0017]FIG. 11 is a block diagram showing a structure of a signalprocessing device 16 a as an example of the signal processing device 16.The signal processing device 16 a includes a first frequencydiscriminator 101, a second frequency discriminator 102, a thirdfrequency discriminator 103 and an arithmetic operation device 104.

[0018] The first frequency discriminator 101 conducts a frequencyanalysis upon receiving a reception signal from the first photo detector27 and extracts a Doppler frequency from a target. The second frequencydiscriminator 102 conducts the frequency analysis of the monitor signaland obtains the frequency difference f_(M) between the laser pulse 21and the local light 25 and the oscillation timing of the laser pulsefrom the monitor signal. The third frequency discriminator 103 transmitsthe error signal based on the value of the frequency difference f_(M)between the laser pulse 21 and the local light 25 to the control circuit18 from the monitor signal. The arithmetic operation device 104calculates a distance and a velocity of the target on the basis of anoutput signal from the first and second frequency discriminators 101 and102.

[0019] In this example, the structure of the frequency discriminatorthat functions as the first frequency discriminator 101, the secondfrequency discriminator 102 and the third frequency discriminator 103includes an A/D converter that converts the reception signal or themonitor signal into a digital signal and a signal processing portionthat processes the digital signal converted by the A/D converter into anecessary signal by a frequency analyzing means such as a fast Fouriertransform (FFT) as shown in FIG. 12.

[0020] Also, as shown in FIG. 13, the frequency discriminator may bemade up of an electric filter portion which is made up of one or aplurality of electric filters, and a signal processing portion thatconducts a necessary signal processing in accordance with thetransmittance of a signal from the electric filter portion.

[0021] Also, the signal processing device 16 can employ a signalprocessing device 16 b that incorporates the function of the thirdfrequency discriminator 103 shown in FIG. 11 into the second frequencydiscriminator 102 shown in FIG. 11, as shown in FIG. 14, and has thesame function as that of the signal processing device 16 a shown in FIG.11.

[0022] As described above, in the photo detecting portion 10 of thecoherent laser radar device using the conventional injection seedingpulsed laser 4 as the light source, the second photo detector 28 formonitoring the oscillation frequency of the injection seeding pulsedlaser 4 is disposed in addition to the first photo detector 27 thatdetects the reception light as shown in the photo detecting portion 10 ashown in FIG. 10. In addition, at least two systems for the receptionsignal, the monitor and so on are prepared for the frequencydiscriminator as shown in FIGS. 11 and 14.

[0023] In the case where the intensity of the monitor light issufficiently large, a part or the entire microwave amplifier of thesecond photo detector 28 can be omitted.

[0024] Subsequently, an influence of an internal reflection light of thecoherent laser radar device using a pulsed laser light source and acoaxial transmission/reception optical system as shown in FIG. 9 will bedescribed.

[0025] In FIG. 9, the beam splitter 5, the ¼ wavelength plate 6, thetelescope 7 and the scanning optical system 8 are of the coaxialtransmission/reception optical system that makes the optical axes 22 ofthe transmit/reception lights substantially coincide with each other.

[0026] In the coherent laser radar device using the coaxialtransmission/reception optical system of this type, the internalreflection lights from the optical elements that constitute the coaxialtransmission/reception optical system reach the photo detecting portion10 through the same path as that through which the reception lightpasses. In particular, since the internal reflection lights from thetelescope 7 and the scanning optical system 8 pass through the beamsplitter 5 as a reception side, the influence of the internal reflectionlight is large. As usual, the attenuation of reflection of the telescope7 and the scanning optical system 8 is about 60 to 70 dB. On thecontrary, the attenuation of reflection of the reception light fromaerosol contained in the atmosphere exceeds 100 dB.

[0027] In order to conduct a high-precision measurement, the microwaveamplifier which is made up of the pre-amplifier and the post-amplifierof the first photo detector 27 within the photo detecting portion 10 ashown in FIG. 10 is required to amplify the reception signal up to abouta degree suitable for the maximum sampling amplitude of the firstfrequency discriminator 101. Since the reception light is slight, themicrowave amplifier of the first photo detector 27 has a high gain.Since the internal reflection light is much larger in power than thereception light, the internal reflection light induces the saturation ofthe microwave amplifier in the first photo detector 27. Since the linearamplification of the signal is not conducted until the microwaveamplifier is restored since the microwave amplifier is saturated, themeasurement cannot be conducted. As usual, it takes several μs until theinfluence of such an internal reflection light is eliminated. For thatreason, a “blind zone” where the short distance of several hundreds of mfrom the device cannot be measured occurs.

[0028] As described above, the coherent laser radar device using theconventional injection seeding pulsed laser 4 shown in FIG. 9 as thelight source and also using the coaxial transmission/reception opticalsystem suffers from the following drawbacks.

[0029] 1. In order to monitor the oscillation frequency of the injectionseeding pulsed laser 4, the photo detector for monitoring is disposed inaddition to the photo detector that detects the reception light,resulting in the complicated photo detecting portion.

[0030] 2. Likewise, at least two systems for the reception signal andthe monitor are disposed for the frequency discriminator with the resultthat the signal processing device is complicated.

[0031] 3. The wide “blind zone” where the measurement cannot beconducted over the short distance of several hundreds of m from thedevice occurs due to the influence of the internal reflection light ofthe transmission/reception optical system.

[0032] The present invention has been made to eliminate theabove-described problems, and therefore an object of the presentinvention is to provide a coherent laser radar device using an injectionseeding pulsed laser as a light source and also using a coaxialtransmission/reception optical system in which a photo detector is ofone system and the blind zone can be eliminated.

DISCLOSURE OF THE INVENTION

[0033] In order to achieve the above-mentioned object, according to thepresent invention, there is provided a coherent laser radar device,comprising: a local light source that oscillates at a single frequency;a pulsed laser that oscillates at a frequency which is identical with orclose to an output light of the local light source as a singlefrequency; a transmission/reception optical system that irradiates apulsed laser beam from the pulsed laser toward a target as atransmission light and receives a scattered light from the target as areception light; a light coupling means that couples the output lightfrom the local light source and the reception light; a photo detectingportion that conducts light coherent detection on the light coupled bythe light coupling means; and a signal processing device that calculatesa speed and a distance of a target in accordance with an output from thephoto detecting portion, characterized in that the photo detectingportion comprises: a photo detecting element that conducts the lightcoherent detection; a microwave switch that changes over a propagationpath of an output from the photo detecting element; a microwaveamplifier; and a switch control means that changes over the microwaveswitch so as to transmit a signal before a reference time to the signalprocessing device as a monitor signal and transmit a signal after thereference time to the signal processing device as a reception signalwith a time at which the pulse light from the pulsed laser hascompletely passed through the transmission/reception optical system asthe reference time.

[0034] Also, the pulsed laser includes an adjusting mechanism thatadjusts a cavity length, the device further comprises a control circuitthat controls the adjusting mechanism, and the control circuit outputsto the adjusting mechanism a control signal that adjusts the cavitylength of the pulsed laser on the basis of an error signal from thesignal processing device based on a frequency difference between thelaser pulse and the local light.

[0035] Also, the microwave amplifier is made up of a pre-amplifier thatamplifies a signal from the photo detecting element and a post-amplifierthat amplifies an output of the pre-amplifier, and the microwave switchis disposed between the pre-amplifier and the post-amplifier, outputs asignal that has been amplified by the pre-amplifier as a monitor signaland outputs a signal that has passed through the post-amplifier as areception signal.

[0036] Also, the microwave amplifier is made up of a pre-amplifier thatamplifies a signal from the photo detecting element and a post-amplifierthat amplifies an output of the pre-amplifier, and the microwave switchis disposed between the photo detecting element and the pre-amplifier,outputs a signal from the photo detecting element as a monitor signaland outputs a signal that has passed through the post-amplifier as areception signal.

[0037] Also, according to another aspect of the present invention, thereis provided a coherent laser radar device, comprising: a local lightsource that oscillates at a single frequency; a pulsed laser thatoscillates at a frequency which is identical with or close to an outputlight of the local light source as a single frequency; atransmission/reception optical system that irradiates a pulsed laserbeam from the pulsed laser toward a target as a transmission light andreceives a scattered light from the target as a reception light; a lightcoupling means that couples the output light from the local light sourceand the reception light; a photo detecting portion that conducts lightcoherent detection on the light coupled by the light coupling means; anda signal processing device that detects a speed and a distance of atarget in accordance with an output from the photo detecting portion,characterized in that the photo detecting portion comprises: a photodetecting element that conducts the light coherent detection; amicrowave amplifying portion that amplifies an output signal from thephoto detecting element; and a gain control means that controls the gainof the microwave amplifying portion so that an amplitude of the outputsignal from the microwave amplifying portion does not exceed a giventhreshold value.

[0038] Also, the pulsed laser includes an adjusting mechanism thatadjusts a cavity length; the device further comprises a control circuitthat controls the adjusting mechanism, and the control circuit outputsto the adjusting mechanism a control signal that adjusts the cavitylength of the pulsed laser on the basis of an error signal from thesignal processing device based on a frequency difference between thelaser pulse and the local light.

[0039] Also, the microwave amplifier is made up of a pre-amplifier thatamplifies a signal from the photo detecting element and a gain controlamplifier that amplifies an output of the pre-amplifier, and the gaincontrol means controls the gain of the gain control amplifier.

[0040] Also, the microwave amplifier is made up of a pre-amplifier thatamplifies a signal from the photo detecting element and a post-amplifierthat amplifies an output of the pre-amplifier, and the gain controlmeans comprises a microwave variable attenuator disposed between thepre-amplifier and the post-amplifier, and an attenuation control circuitthat controls the attenuation of the microwave variable attenuator.

[0041] Also, a coherent laser radar device according to still anotheraspect of the present invention is characterized by comprising: a locallight source that oscillates at a single frequency; a pulsed laser thatoscillates at a frequency which is identical with or close to an outputlight of the local light source as a single frequency; atransmission/reception optical system that irradiates a pulsed laserbeam from the pulsed laser toward a target as a transmission light andreceives a scattered light from the target as a reception light; a lightcoupling means that couples the output light from the local light sourceand the reception light; a photo detecting portion that conducts lightcoherent detection on the light coupled by the optical coupler; and asignal processing device that detects a speed and a distance of a targetin accordance with an output from the photo detecting portion; a lightvariable attenuator disposed between the transmission/reception opticalsystem and the photo detecting portion; and a control means thatcontrols the attenuation of the light variable attenuator in such amanner that an amplitude of the output from the photo detecting portiondoes not exceed a given threshold value.

[0042] Further, the pulsed laser includes an adjusting mechanism thatadjusts a cavity length, the device further comprises a control circuitthat controls the adjusting mechanism, and the control circuit outputsto the adjusting mechanism a control signal that adjusts the cavitylength of the pulsed laser on the basis of an error signal from thesignal processing device based on a frequency difference between thelaser pulse and the local light.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043]FIG. 1 is a block diagram showing a structure of a coherent laserradar device in accordance with a first embodiment of the presentinvention;

[0044]FIG. 2 is a block diagram showing a structure of a photo detectingportion 10Aa employed as a photo detecting portion 10A in the firstembodiment of the present invention;

[0045]FIG. 3 is a block diagram showing a structure of a photo detectingportion 10Ab employed as a photo detecting portion 10A in the firstembodiment of the present invention;

[0046]FIG. 4 is a block diagram showing a structure of a photo detectingportion 10B in accordance with a second embodiment of the presentinvention;

[0047]FIG. 5 is a block diagram showing a structure of a signalprocessing device 16A used as a signal processing device 16 inaccordance with the second embodiment of the present invention;

[0048]FIG. 6 is a block diagram showing a structure of a photo detectingportion 10C in accordance with a third embodiment of the presentinvention;

[0049]FIG. 7 is a block diagram showing a structure of a coherent laserradar device in accordance with a fourth embodiment of the presentinvention;

[0050]FIG. 8 is a block diagram showing a structure of a photo detectingportion 10D and an optical system immediately in front of the photodetecting portion 10D in the fourth embodiment of the present invention;

[0051]FIG. 9 is a block diagram showing a structure of a laser radardevice in which a coherent laser radar device using an injection seedingpulsed laser device as a light source as disclosed in U.S. Pat. No.5,237,331 B is combined with a wavelength synchronizing circuit forstabilizing the wavelength of a laser radar light source as disclosed inJP 10-54760 A;

[0052]FIG. 10 is a block diagram showing a structure of a photodetecting portion 10 a in accordance with a conventional example;

[0053]FIG. 11 is a block diagram showing a structure of a signalprocessing device 16 a as an example of a signal processing device 16 inaccordance with a conventional example;

[0054]FIG. 12 is a block diagram showing an example of a structure of afrequency discriminator in FIG. 11;

[0055]FIG. 13 is a block diagram showing another example of thestructure of the frequency discriminator in FIG. 11; and

[0056]FIG. 14 is a block diagram showing the structure of a signalprocessing device 16 b as an example of a signal processing device 16 inthe conventional example.

BEST MODES FOR CARRYING OUT THE INVENTION

[0057] First Embodiment

[0058]FIG. 1 is a block diagram showing a structure of a coherent laserradar device in accordance with a first embodiment of the presentinvention. In FIG. 1, the same parts as those in the conventionalexample shown in FIG. 9 are designated by like reference numerals andtheir description will be omitted.

[0059] In the coherent laser radar device shown in FIG. 1, differencesfrom the conventional example shown in FIG. 9 will be described. First,there is disposed a photo detecting portion 10A that is different in theinternal structure from the conventional example as will be describedlater. Also, there is disposed a coupling optical system 29 between thebeam splitter 5 and the first optical coupler 9. In addition, the laserlight source 1, the first optical divider 2, the frequency shifter 3 andthe injection seeding pulsed laser 4 are connected to each other in thestated order by optical fibers, the first optical divider 2 and thefirst optical coupler 9 are connected to each other by an optical fiber,and the coupling optical system 29, the first optical coupler 9 and thephoto detecting portion 10A of the first embodiment are connected toeach other by optical fibers in the stated order. Then, the receptionlight 24 is coupled to the optical fiber by the coupling optical system29 and then coupled with the local light 25 in the fiber inline typeoptical coupler 9. The coupled light 26 consisting of the receptionlight 24 and the local light 25 passes through the optical fiber and arethen supplied to the photo detecting portion 10A.

[0060] On the other hand, the same portions as those in the conventionalexample shown in FIG. 9 will be described. A process that the injectionseeding pulsed laser (hereinafter simply referred to as pulsed laser) 4oscillates at a single frequency and a process that an output laserpulse from the injection seeding pulsed laser is transmitted toward atarget as a transmission light and a scattered light from the target isreceived are identical with those in the conventional example shown inFIG. 9. Also, a process from supplying of the monitor signal and thereception signal to the signal processing device 16 from the photodetecting portion 10A of the first embodiment to the signal processingin the signal processing device 16 are also identical with those in theconventional example shown in FIG. 9. The signal processing device 16may be structured by the signal processing device 16 a shown in FIG. 11or the signal processing device 16 b shown in FIG. 14.

[0061]FIG. 2 is a block diagram showing a structure of a photo detectingportion 10Aa employed as a photo detecting portion 10A in the firstembodiment of the present invention. The photo detecting portion 10Aashown in FIG. 2 is made up of a photodiode 30, a pre-amplifier 31, apost-amplifier 32, a microwave switch 33 and a switch control circuit 34that controls the microwave switch 33.

[0062] The microwave switch 33 is controlled by the switch controlcircuit 34 as follows. While an internal reflection light is receivedfrom the transmission/reception optical system to the photodiode 30before the pulsed laser 4 oscillates, the microwave switch 33 transmitsa signal from the photodiode 30 which has been amplified by thepre-amplifier 31 to the signal processing device 16 as the monitorsignal. After the internal reflection light has been sufficientlyattenuated, the microwave switch 33 transmits a signal from thephotodiode 30 to the signal processing device 16 through thepost-amplifier 32 as the reception signal. That is, the switch controlcircuit 34 changes over the microwave switch so as to transmit a signalbefore a reference time to the signal processing device 16 as themonitor signal and transmit a signal after the reference time to thesignal processing device 16 as the reception signal with a time at whichthe pulse light from the pulsed laser 4 has completely passed throughthe transmission/reception optical system as the reference time.

[0063] Since the internal reflection light is also a part of the outputlaser pulse of the pulsed laser 4, the light coherent detection signalcan be employed as a monitor signal for obtaining the frequencydifference between the laser pulse 21 and the local light 25 and theoscillation timing of the laser pulse. The microwave switch 33 changesover after the internal reflection light has been sufficientlyattenuated, thereby being capable of transmitting the reception signalwithout saturating the post-amplifier 32.

[0064] In addition, if a timing at which the microwave switch 33 changesover is a time point at which a signal resulting from the internalreflection light is attenuated to the degree at which the post-amplifier32 is not saturated, the “blind zone” can be reduced. For example, ifthe pulse width of the laser pulse 21 is 200 ns, it is possible to setthe change-over timing to 1 μs or less after oscillation. That is, the“blind zone” can be reduced to 150 m or less.

[0065] If the peak intensity of the internal reflection light issufficiently high, the microwave switch 33 may be disposed between thephotodiode 30 and the pre-amplifier 31 as in the photo detecting portion10Ab shown in FIG. 3. In this situation, the timing at which themicrowave switch 33 changes over is a point of time where the internalreflection light attenuates to the degree at which the pre-amplifier 31and the post-amplifier 32 are not saturated.

[0066] The microwave switch 33 is required to provide the switchingspeed of about 0.1 μs and sufficient In-Out isolation that does notsaturate the amplifier when the signal is off. However, in asemiconductor switch using GaAs or the like, the switch speed of 10 nsand the In-Out isolation of 40 dB or more are realized, and themicrowave switch 33 can be formed of the semiconductor switch.

[0067] Since the above-mentioned structure makes it possible that themicrowave switch 33 changes over to produce the monitor signal by usingthe internal reflection light, it is unnecessary to additionally providean optical system for extracting a part of the laser pulse 24 and aphoto detector for producing the monitor signal with the result that thedevice can be simplified. Also, the “blind zone” can be reduced by thechange-over timing of the microwave switch 33 without saturating thepre-amplifier 31 and the post-amplifier 32.

[0068] Second Embodiment

[0069]FIG. 4 is a block diagram showing a structure of a photo detectingportion 10B in accordance with a second embodiment of the presentinvention. The photo detecting portion 10B in the second embodiment ofthe present invention is employed instead of the photo detecting portion10A according to the first embodiment in FIG. 1 and includes aphotodiode 30, a pre-amplifier 31, a gain control amplifier 35 and again control circuit 36 that controls the gain of the gain controlamplifier 35.

[0070] The pre-amplifier 31 and the gain control amplifier 35 structurethe microwave amplifier in the photo detecting portion 10B, and the gaincontrol amplifier 35 and the gain control circuit 36 structure the gaincontrol means of the microwave amplifier.

[0071] Also, FIG. 5 is a block diagram showing a signal processingdevice 16A used as a signal processing device 16 in accordance with thesecond embodiment of the present invention. Since the signal processingdevice 16A in the second embodiment of the present invention is employedalso as the signal processing device 16 shown in FIG. 1, the signalprocessing device 16A is made up of only a frequency discriminator 110and an arithmetic operation device 104.

[0072] In the second embodiment, a process that the injection seedingpulsed laser 4 oscillates at a single frequency and a process that anoutput laser pulse from the injection seeding pulsed laser istransmitted toward a target as a transmission light and a scatteredlight from the target is received and supplied to the photo detectingportion 10B are identical with those in the first embodiment. The secondembodiment is characterized in that the gain of the microwave amplifierin the photo detecting portion 10B is controlled in accordance with atime. The gain of the microwave amplifier is set to be low so that thepeak value of a signal amplitude resulting from the internal reflectionlight does not exceed the sampling maximum amplitude of the frequencydiscriminator 110 while the internal reflection light is received by thephotodiode 30 from the transmission/reception optical system.Thereafter, after the internal reflection light has been sufficientlyattenuated, the gain of the microwave amplifier is set to a high gainsufficient to amplify the reception signal to the degree suitable forthe sampling maximum amplitude of the frequency discriminator 110.

[0073] As described above, the gain of the microwave amplifier iscontrolled in such a manner that the monitor signal can enter a timezone of the reception signal which has not been conventionally used inthe signal processing as the “blind zone”. Since the monitor signal andthe reception signal are allowed to flow in one signal line in a timedivision manner, two systems of the photo detectors and the frequencydiscriminators which have been required as the reception signal and themonitor signal up to now can be simplified into one system. In addition,there is advantageous in that the “blind zone” can be reduced due to thechange-over timing of the gain of the microwave amplifier as in thefirst embodiment.

[0074] The above-mentioned structures make it possible to obtain anadvantage that two systems of the photo detectors and the frequencydiscriminators which have been required as the reception signal and themonitor signal up to now can be simplified into one system and anadvantage that the “blind zone” can be reduced due to the change-overtiming of the gain of the microwave amplifier as in the firstembodiment.

[0075] Third Embodiment

[0076]FIG. 6 is a block diagram showing a photo detecting portion 10C inaccordance with a third embodiment of the present invention. The photodetecting portion 10C in the third embodiment of the present inventionis used instead of the photo detecting portion 10A according to thefirst embodiment in FIG. 1 and includes a photodiode 30, a pre-amplifier31, a post-amplifier 32, a microwave variable attenuator 37 and anattenuation control circuit 38 that controls the attenuation of thevariable attenuator 37.

[0077] In other words, in the photo detecting portion 10C in the thirdembodiment, the microwave variable attenuator 37 is inserted between thepre-amplifier 31 and the post-amplifier 32 to structure the microwaveamplifier of the photo detector. Then, the attenuation control circuit38 controls the attenuation of the variable attenuator 37.

[0078] As a result, the monitor signal and the reception signal areallowed to flow in one output signal line in a time division manner.

[0079] The signal processing device 16 in the third embodiment of thepresent invention employs the same signal processing device 16A as thatin the second embodiment shown in FIG. 5.

[0080] The above-mentioned structure makes it possible to obtain anadvantage that two systems of the photo detectors and the frequencydiscriminators which have been required as the reception signal and themonitor signal up to now can be simplified into one system and anadvantage that the “blind zone” can be reduced due to the change-overtiming of the gain of the microwave amplifier as in the firstembodiment.

[0081] The variable attenuator 37 can be formed of a semiconductorswitch using GaAs or the like.

[0082] Fourth Embodiment

[0083]FIG. 7 is a block diagram showing the structure of a coherentlaser radar device in accordance with a fourth embodiment of the presentinvention. In FIG. 7, the same parts as those in the first embodimentshown in FIG. 1 are designated by like references and their descriptionwill be omitted. In the coherent laser radar device according to thefourth embodiment, a variable light attenuator 39 and an attenuationcontrol circuit 40 that controls the attenuation of the variable lightattenuator 39 are further disposed between the coupling optical system27 and the first optical coupler 9 with respect to the first embodimentshown in FIG. 1, as shown in FIG. 7. Also, there is disposed a photodetecting portion 10D different in the internal structure from those inthe first to third embodiments as will be described later. The signalprocessing device 16 in the fourth embodiment of the present inventionis formed of a signal processing device 16A like to that in the secondembodiment shown in FIG. 5.

[0084]FIG. 8 is a block diagram showing the structure of a photodetecting portion 10D and an optical system immediately in front of thephoto detecting portion 10D in the fourth embodiment of the presentinvention.

[0085] As shown in FIG. 8, in the fourth embodiment, the photo detectingportion 10D structures a pair of photo detectors formed of thecombination of a photodiode 30 that is a photo detecting element whichconducts coherent detection with a microwave amplifier which is formedof a pre-amplifier 31 and a post-amplifier 32 and has a fixed gain.

[0086] In this embodiment, the function of the structure of the secondembodiment shown in FIGS. 4 and 5 is achieved by controlling theattenuation of the reception light 24 by the variable light attenuator39 and the attenuation control circuit 40. That is, in the secondembodiment, the function with which an output signal from the photodetector 10B to the signal processing device 16B does not exceed thesampling maximum amplitude of the frequency discriminator 110 by thegain control circuit 36 of the microwave amplifier in the photo detector10B even while the internal reflection light is received by thephotodiode 30 from the transmission/reception optical system is achievedby controlling the attenuation of the reception light 24 by the variablelight attenuator 39 and the attenuation control circuit 40.

[0087] The attenuation of the variable light attenuator 39 is set to behigh so that a peak value of a signal amplitude resulting from theinternal reflection light does not exceed the sampling maximum amplitudeof the frequency discriminator 110 shown in FIG. 5 while the internalreflection light is received by the photodiode 30 shown in FIG. 8 fromthe transmission/reception optical system, to thereby limit the power ofthe internal reflection light received by the photodiode 30. Thereafter,the internal reflection light has been sufficiently attenuated, controlis then made in such a manner that the attenuation of the variable lightattenuator 39 is set to 0 or nearly 0, and the power loss of thereception light which is received by the photodiode 30 is lowered. Thegain of the microwave amplifier is set to sufficient high to amplify thereception signal at that time to the degree suitable for the samplingmaximum amplitude of the frequency discriminator 110.

[0088] As described above, the attenuation of the variable lightattenuator 39 is controlled, to thereby make it possible that themonitor signal enters the time zone of the reception signal that has notbeen conventionally used in the signal processing as the “blind zone” asin the second embodiment. Since the monitor signal and the receptionsignal are allowed to flow in one signal line in a time division manner,two systems of the photo detectors and the frequency discriminatorswhich have been required as the reception signal and the monitor signalup to now can be simplified into one system. In addition, there isadvantageous in that the “blind zone” can be reduced due to thechange-over timing of the gain of the microwave amplifier as in thesecond embodiment.

[0089] The above-mentioned structure makes it possible to obtain anadvantage that two systems of the photo detectors and the frequencydiscriminators which have been required as the reception signal and themonitor signal up to now can be simplified into one system and anadvantage that the “blind zone” can be reduced due to the change-overtiming of the gain of the microwave amplifier as in the firstembodiment.

[0090] The above-mentioned first to fourth embodiments show thestructural examples in which the laser light source 1, the first opticaldivider 2, the frequency shifter 3 and the injection seeding pulsedlaser 4 are coupled to each other by optical fibers in the stated order,respectively, the first optical divider 2 and the first optical coupler9 are coupled to each other by an optical fiber, and the couplingoptical system 29, the first optical coupler 9, the photo detectingportion 10 (10A, 10Aa, 10Ab, 10B, 10C, 10D) are coupled to each other byoptical fibers in the stated order, respectively, so as to allow thelocal light 25, the reception light 24, the seed light 20 and thecoupled light 26 to be propagated in the optical fiber.

[0091] The first to fourth embodiments may be structured in such amanner that all or a part of the coupling optical system 29 and theoptical fiber are omitted as in the conventional example shown in FIG.9, and all or a part of the local light 25, the reception light 24, theseed light 20 and the coupled light 26 are propagated into a space, andthe same advantages as those described above can be obtained.

INDUSTRIAL APPLICABILITY

[0092] As was described above, according to the present invention, inthe coherent laser radar device that uses the injection seeding pulsedlaser as a light source and also uses the coaxial transmission/receptionoptical system, there can be obtained the coherent laser radar devicewhich is capable of simplifying the photo detectors of two systems forthe reception signal and the monitor signal into one system, and is alsocapable of reducing the blind zone.

1. A coherent laser radar device comprising: a local light source thatoscillates at a single frequency; a pulsed laser that oscillates at afrequency which is identical with or close to an output light of saidlocal light source as a single frequency; a transmission/receptionoptical system that irradiates a pulsed laser beam from said pulsedlaser toward a target as a transmission light and receives a scatteredlight from the target as a reception light; a light coupling means thatcouples the output light from said local light source and said receptionlight; a photo detecting portion that conducts light coherent detectionon the light coupled by said light coupling means; and a signalprocessing device that calculates a speed and a distance of a target inaccordance with an output from said photo detecting portion,characterized in that said photo detecting portion comprises: a photodetecting element that conducts the light coherent detection; amicrowave switch that changes over a propagation path of an output fromsaid photo detecting element; a microwave amplifier; and a switchcontrol means that changes over said microwave switch so as to transmita signal before a reference time to said signal processing device as amonitor signal and transmit a signal after the reference time to saidsignal processing device as a reception signal with a time at which thepulse light from said pulsed laser has completely passed through saidtransmission/reception optical system as the reference time.
 2. Acoherent laser radar device according to claim 1, characterized in that:said pulsed laser includes an adjusting mechanism that adjusts a cavitylength; said device further comprises a control circuit that controlssaid adjusting mechanism; and said control circuit outputs to saidadjusting mechanism a control signal that adjusts the cavity length ofsaid pulsed laser on the basis of an error signal from said signalprocessing device based on a frequency difference between the laserpulse and the local light.
 3. A coherent laser radar device according toclaim 1, characterized in that: said microwave amplifier is made up of apre-amplifier that amplifies a signal from said photo detecting elementand a post-amplifier that amplifies an output of the pre-amplifier; andsaid microwave switch is disposed between said pre-amplifier and saidpost-amplifier, outputs a signal that has been amplified by saidpre-amplifier as a monitor signal and outputs a signal that has passedthrough said post-amplifier as a reception signal.
 4. A coherent laserradar device according to claim 1, characterized in that: said microwaveamplifier is made up of a pre-amplifier that amplifies a signal fromsaid photo detecting element and a post-amplifier that amplifies anoutput of the pre-amplifier; and said microwave switch is disposedbetween said photo detecting element and said pre-amplifier, outputs asignal from said photo detecting element as a monitor signal and outputsa signal that has passed through said post-amplifier as a receptionsignal.
 5. A coherent laser radar device comprising: a local lightsource that oscillates at a single frequency; a pulsed laser thatoscillates at a frequency which is identical with or close to an outputlight of said local light source as a single frequency; atransmission/reception optical system that irradiates a pulsed laserbeam from said pulsed laser toward a target as a transmission light andreceives a scattered light from the target as a reception light; a lightcoupling means that couples the output light from said local lightsource and said reception light; a photo detecting portion that conductslight coherent detection on the light coupled by said light couplingmeans; and a signal processing device that calculates a speed and adistance of a target in accordance with an output from said photodetecting portion, characterized in that said photo detecting portioncomprises: a photo detecting element that conducts the light coherentdetection, a microwave amplifying portion that amplifies an outputsignal from said photo detecting element; and a gain control means thatcontrols the gain of said microwave amplifying portion so that anamplitude of the output signal from said microwave amplifying portiondoes not exceed a given threshold value.
 6. A coherent laser radardevice according to claim 5, characterized in that: said pulsed laserincludes an adjusting mechanism that adjusts a cavity length; saiddevice further comprises a control circuit that controls said adjustingmechanism; and said control circuit outputs to said adjusting mechanisma control signal that adjusts the cavity length of said pulsed laser onthe basis of an error signal from said signal processing device based ona frequency difference between the laser pulse and the local light.
 7. Acoherent laser radar device according to claim 5, characterized in that:said microwave amplifier is made up of a pre-amplifier that amplifies asignal from said photo detecting element and a gain control amplifierthat amplifies an output of the pre-amplifier; and said gain controlmeans controls the gain of said gain control amplifier.
 8. A coherentlaser radar device according to claim 5, characterized in that: saidmicrowave amplifier is made up of a pre-amplifier that amplifies asignal from said photo detecting element and a post-amplifier thatamplifies an output of the pre-amplifier; and said gain control meanscomprises a microwave variable attenuator disposed between saidpre-amplifier and said post-amplifier, and an attenuation controlcircuit that controls the attenuation of said microwave variableattenuator.
 9. A coherent laser radar device characterized bycomprising: a local light source that oscillates at a single frequency;a pulsed laser that oscillates at a frequency which is identical with orclose to an output light of said local light source as a singlefrequency; a transmission/reception optical system that irradiates apulsed laser beam from said pulsed laser toward a target as atransmission light and receives a scattered light from the target as areception light; a light coupling means that couples the output lightfrom said local light source and said reception light; a photo detectingportion that conducts light coherent detection on the light coupled bysaid optical coupler; and a signal processing device that detects aspeed and a distance of a target in accordance with an output from saidphoto detecting portion; a light variable attenuator disposed betweensaid transmission/reception optical system and said photo detectingportion; and a control means that controls the attenuation of said lightvariable attenuator in such a manner that an amplitude of the outputfrom said photo detecting portion does not exceed a given thresholdvalue.
 10. A coherent laser radar device according to claim 9,characterized in that: said pulsed laser includes an adjusting mechanismthat adjusts a cavity length; said device further comprises a controlcircuit that controls said adjusting mechanism; and said control circuitoutputs to said adjusting mechanism a control signal that adjusts thecavity length of said pulsed laser on the basis of an error signal fromsaid signal processing device based on a frequency difference betweenthe laser pulse and the local light.